Earth formation drilling is often accomplished using a long string of drilling pipes and tools coupled together. The drilling string is rotated together in order to rotate a cutting bit at the end of the string. This cutting bit creates the hole which the rest of the drilling string moves through. For various reasons, it may be desirable to widen the walls of the hole after it has been created by the cutting bit. Bore-hole underreamers exist to accomplish the widening of the hole. An underreamer may be coupled to the drilling string between two other elements of the drilling string. It may then be sent down hole with the drilling string, rotating with the drilling string, and widening the hole.
Various underreamer designs exist. Some have fixed cutting blades around the periphery of the underreamer and some have expandable blades or arms. Various types and hardness of earth formations also exist. Aggressive blades, extending quickly and/or relatively far beyond the periphery of the underreamer body, may be used in soft formations; and less aggressive blades, extending more slowly and/or a shorter distance beyond the periphery of the underreamer body, may be used in harder formations. Different types of formations may exist down the length of a drilling hole, and it may be desirable to widen the hole through each of these formation types. If the blades or arms with which the underreamer is equipped are not suitable for the types of formations being widened, the underreamer may need to be replaced. This generally involves pulling the drill string up from the hole, disconnecting the underreamer, and connecting an underreamer equipped with blades or arms that are suitable for the formation type. This may require a drilling operator to have several underreamers on hand as well as the tools required to change underreamers. The increased inventory requires a greater capital investment, more storage space, and greater maintenance costs than having a single underreamer.
Over the lifetime of the underreamer the blades or arms of the underreamer may become worn. When the underreamer is no longer able to perform a widening of the drilling hole, it may be withdrawn from the drilling hole and disconnected from the drilling string. A new underreamer may be put in its place, and the worn underreamer may be sent for retooling and refurbishment. Sending the worn underreamer away for retooling and refurbishment may result in costly down time or increased inventory and maintenance costs by requiring a replacement underreamer to be kept available.